The overall goal of this proposal is to evaluate the role of neuregulin-1 (NRG-1) receptors in the initiation and propagation of malignant peripheral nerve sheath tumors (MPNSTs). The first specific aim will test the hypothesis that erbB3 and erbB4 are required for MPNST tumorigenesis by knocking down these receptors in vitro and knocking out these receptors in vivo. The second aim will test the hypothesis that neurofibromas and MPNSTs arise from a novel, erbB4-positive precursor population within the dorsal root ganglion (DRG). The rationale for these investigations stems from recent studies in which erbB4-positive progenitor cells have been isolated from adult mouse DRGs and previous research in our laboratory showing that inhibition of erbB receptors results in decreased tumor migration and proliferation. These data indicate the importance of erbB signaling in tumor pathogenesis and suggest that erbB receptors may be a candidate for targeted therapeutics. In the first specific aim, erbB 3 and erbB4 will be knocked down in human MPNST cell lines using small interfering RNA and the effects on migration, proliferation and survival will be analyzed. In addition, we will cross our mice that develop neurofibromas and MPNSTs to mice lacking erbB3 or erbB4 and monitor the animals for tumor development to assess the importance of NRG-1 receptors to MPNST tumorigenesis in vivo. For the second aim, we will determine if neurofibromas and MPNSTs arise from an erbB4-positive precursor using two methods. First, we will cross erbB4-CreERT2 mice carrying a Cre activatable Tomato (Td) reporter to Nf1flox/- mice and then ablate Nf1 in these cells. Secondly, we will also establish whether these progenitors give rise to MPNSTs by isolating erbB4-positive progenitors from the ganglia of P0-GGFB3; p53flox/flox; Td mice and then ablating p53 with an adenovirus-mediated Cre recombinase. The resulting cells will be allografted into athymic nude mice and then monitored for tumor development. Relevance to public health: MPNSTs are aggressive tumors that commonly arise in the dorsal root ganglia of patients with neurofibromatosis type-1 (NF1). The main treatment for patients with these tumors is surgery and few options exist for patients in which the tumor cannot be excised in its entirety. The work proposed here will attempt to isolate the tumor initiating cells and understand the key signaling cascades required for tumor development in order to identify potential chemotherapeutic targets.